Figure 2. Structural evolution at the molecular level from compound 1o at 123 K to compound 1t at 438 K.

(a) The interlocking angle ω changing from 72.6° to 90°; (b) scheme showing the structural changes; (c) structures showing BPE acting as a molecular spring along the a and b axes via pyridyl ring-flipping; (d) structures showing H₄BPTC lying down toward the a axis as a molecular rotor; and (e) scheme illustrating the dihedral angles δ and κ between pyridyl rings in BPE changing significantly because of pyridyl ring-flipping.